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8.
COMMNICATIONS USING DEDICATED PROTOCOLS
MELSEC-A
(3)
Special-function module buffer memory
The special-function module buffer memory is comprised of 16-bit (one
word) addresses. Read and write of the special-function module buffer
memory is executed by TO and FROM instructions transmitted between
the PC CPU and special-function module.
When the computer reads from and writes t o the special-function
module buffer memory via the AJ71C24, it is done in byte units (1
address
= 8
bits).
The addresses specified in the computer (hexadecimal) are converted
from FROMITO instruction addresses as shown below:
Designated address (hexadecimal)
=
Module head address
+
[(FROMITO instruction address x 2) converted into hexadecimal]
Example: To designate AD61 high-speed counter module FROMITO in-
struction address 1 (CH.l preset value).
Specified address
=
FROM/TO instruction address 1
x
2
+
Head address
8 2 H
2 H
80H
The data format when the computer makes a read or write to or from the
special-function module buffer memory via the AJ71 C24, is explained below
using the AD61 module as an example.
Message format at computer
AD61 buffer memory
AD 61 buffer memoly
viewed from the PC CPU
viewed from the computer
!
POINT
The buffer memory of each special-function module has its read and write
area, read-only and write-only areas, and areas reserved for OS use,
which are not available to the use. See the manual for each module
before using the buffer memory.
I
PC CPU or special-function module errors may occur if reading or writing
is not done correctly.
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